Sealing rings such as O-ring seals or the like have been extensively used in rotary plug valves for sealing off the flow of fluid when the valve is moved to the closed position. These sealing rings have been provided either in a groove on the surface of the plug member of the valve or in a groove in the valve seat of the housing member.
When the sealing ring is provided in a groove on the surface of the valve plug member, the groove and its sealing ring are usually arranged to encircle the inlet port of the housing member when the valve is in the closed position, the theory behind such positioning of the sealing ring being that the upstream fluid pressure acting against the unsupported portion of the sealing ring crossing the inlet port of the valve housing member assists in holding the sealing ring in its groove. Problems of clipping the sealing ring have been encountered with such arrangements, especially when flow through the plug valve was in the reverse direction so that the sealing ring instead of encircling the inlet port was in fact encircling the outlet port. Many efforts have been made to solve the problem of the clipping of the sealing ring when it is positioned in a groove on the surface of the plug member, these efforts including arrangements to cam the sealing ring into its groove or to relieve the pressure differential across the unsupported part of the sealing ring.
When the sealing ring is located in a groove, encircling one of the ports in the housing member of a rotary plug valve it was usually arranged to encircle the downstream port opening, the theory being that the pressure of fluid through the valve was always acting on the sealing ring in a direction tending to hold the sealing ring in its groove. These types of plug valves have been successfully used for many years, wherein the valve is used to either fully open or close flow of fluid in the fluid line. However, these valves were not particularly successful wherein the valve was used as a "throttling valve" as the flow through the valve when in the throttle position created a low pressure area around the unsupported portion of the sealing ring causing it to lift slightly from its seat so that it could be clipped upon rapid closing. Also, this type of valve could not be used as a two-way valve wherein the sealing ring encircles the upstream port opening of the housing member in certain flow conditions and the downstream port opening in other flow conditions. If the sealing ring encircled the inlet or upstream port opening of the housing member and the plug member is being moved to the fully closed position, a rapid flow of fluid through the plug member causes a critical condition to occur just before closing as it creates a low pressure area about the unsupported portion of the sealing ring and, if there is high pressure beneath the sealing ring, the same is lifted from its groove and is clipped. If the sealing ring encircled the downstream or outlet port opening of the housing member and the valve was used as a "throttling valve," there was also present the danger of clipping of the sealing ring upon full closing of the valve.
In an effort to make the rotary plug valve a two-way valve and to eliminate some of the problems mentioned above, either the plug member or the valve seat in the valve housing member was provided with two sealing rings. If the plug was provided with two sealing rings which would surround both the inlet port opening and the outlet port opening of the housing member when the valve was closed, it was thought that this would eliminate the problem of leaking since the arrangement would insure that there would always be a sealing ring surrounding the inlet port regardless of which way the valve assembly was inserted into the fluid pressure line. This same reasoning applied when two sealing rings were carried in the valve seat, one surrounding one port opening and the other surrounding the other port opening. However, these types of arrangements were not entirely satisfactory because the one of the sealing rings not arranged in the desired location would be subject to clipping during operation of the valve and this would interfere with the proper opening and closing of the valve.
The problem of clipping or shearing of sealing rings in rotary plug valves for fluid distribution lines such as gas, water, chemicals or the like has been a constant source of trouble in the industry and many efforts have been made to try and obviate the problem. The present invention which will appear later in the specification under the heading "BRIEF SUMMARY OF THE INVENTION" as well as "DETAILED DESCRIPTION OF THE INVENTION," provides a solution which makes rotary plug valves completely serviceable for flow of fluid in both directions without clipping or shearing of the sealing ring. The present invention also results in a rotary plug valve which may not only be used as an on or off valve but also as a throttling valve.